(170f) In-Situ 3D Printing (IS-3DP) to Enable Adaptive Micropillar Arrays for Clogging-Free Microfluidic Microparticle Filtration

The development of 3D printing technologies has introduced a new era in microfabrication, enabling complex designs inaccessible to conventional soft lithography methods. Among various 3D printing techniques, Digital Light Processing (DLP) has become a popular method to fabricate micro-scale structures, greatly due to its capability to create high-resolution features (tens of micrometers) at a much lower cost. For example, DLP-based techniques have enabled the fabrication of microfluidic devices with complex 3D architectures, including integrated fluid-actuating components such as on-chip pumps and valves. DLP 3D printing relies on the layer-by-layer selective photopolyermization of photocrosslinkable resins in a bath tank; the unpolymerized resin is drained afterwards to complete the print. Therefore, embedding functional microstructures, such as pillar arrays within an void structure (i.e. a microchannel) is particularly challenging due to the difficulty to completely wash out unpolymerized resin from the print. Therefore, the ability to fabricate adaptive, complex, and functional microstructures directly within the existing void structure such as a microchannel presents a transformative opportunity for advancing microfluidic device design and fabrication using DLP-based 3D printing.

Here, we introduce a novel approach termed in-situ 3D printing (IS-3DP) that enables the layer-by-layer fabrication of microstructures inside a microchannel using multi-phase laminar flow. Unlike traditional DLP 3D printing methods which rely on resin VATs and mechanical Z-stage movement, our approach utilizes a flow-defined aqueous two-phase system (ATPS) to delineate each printed layer. By dynamically controlling the thickness of a photopolymerizable phase (PEG resin) flowing adjacent to a non-photopolymerizable phase (Dextran aqueous solution), three-dimensional structures can be formed directly within an enclosed microchannel. The phase boundary in the ATPS consisting of PEG and Dextran solution has an ultra-low interfacial surface tension (less than 500 micro-Newton per millimeter), which ensures the smoothness of the interface, i.e. the printing surface. Since the printing is performed within a microchip and the use of resin VAT is eliminated, we are able to integrate a UV camera opposite the light engine to allow for imaging of the projected pattern on each layer during the printing process. The in operando imaging enables the real-time feedback control to couple the dynamic printing layer with the light projection focal plane to enhance the printing resolution. Uncured resin is replaced directly in the fluidic environment during printing; therefore, the challenge of draining unpolymerized resin from high-resolution features is resolved.

To demonstrate the potential and versatility of IS-3DP, we design and fabricate an adaptive micropillar array integrated in a microchannel for microfluidic-based microparticle filtration. Microfluidic filtration based on particle sizes plays a pivotal role in isolating bioparticles, such as cells and bacteria, from physiological and environmental samples in cell and microbe analysis. However, due to its small size, microfluidic filtration system is more prone to failure due to particle clogging in the channel. By leveraging multiphase flow profiles and precisely controlled photopolymerization, we show that IS-3DP can modulate pillar spacing and geometry, creating adaptive and asymmetric pillar geometry in the Z-direction and enabling selective passage based on particle size while allow clog-free fluid passage throughout the entire channel. We study the size, geometry, and spacing parameters of the adaptive micropillar array to optimize the filtration efficiency and energy consumption and demonstrate the application of this microfluidic filtration device in separating biological cells and standard polystyrene microparticles.